This invention relates to the art of position and velocity sensing and control, and more particularly to a new and improved encoder system for achieving high resolution and low cost.
One area of use of the present invention is in closed loop feedback motor control systems although the principles of the present invention can be variously applied. When driving a load in a servo mechanism, precise control of motor output is achieved by employing closed loop feedback to regulate speed and shaft position to respond to torque perturbations in the system. Accurate control, particularly at very low rpm, requires the use of velocity and positional feedback.
Velocity information is usually provided by a tachometer attached to the motor. However, where 1% or better speed regulation is required relative to the motor tachometer and load, bearing and coupling anomalies, brush friction, and added inertia can become significant impediments to torque-starved low speed servo systems. In particular, such items can account for a large percentage of the motor's output torque. Such systems may also encounter signal to noise problems generated by commutator ripple, which is a ripple on the motor back emf which can cause cogging and velocity modulation.
Positional feedback is frequently provided by an incremental encoder attached to the motor, as in the case of a direct drive system, or to the output shaft in a system using some form of reduction between the motor and load. Encoders with quadrature output provide positional as well as directional information. While potentially not adding significantly to system inertia, encoders capable of positional accurance of less than 0.01% error in a low speed system require high pulse counts. Such encoders are expensive.
It would, therefore, be highly desirable to provide an encoder system and method for low speed operation which achieves high resolution and at the same time is economical.